When oil is discovered it is not uncommon for more than one reservoir to be discovered, with one below the other. This may have been because they were formed at different times in history or because at some time oil was able to move up through a permeable layer which later moved and stopped the flow.
To reduce the costs of recovering oil from each reservoir, a single oil well may be created that passes through each reservoir so that oil can be recovered from each simultaneously. This method of creating two or more producing wells from a single casing is called a dual- or multi-completion well. A diagram of such a well is shown in FIG. 1, which extends below the surface (S) of the ground and intercepts both reservoirs R1 and R2.
As the two reservoirs have developed in different environments and at different times, it is likely that they will have different characteristic pressures and temperatures which can compromise the extraction process. This can be exacerbated over time as the volume of oil remaining in one of the wells may reduce much quicker, and therefore the pressure will drop quicker leading to a lower rate of oil production.
Engineers have developed several tools to overcome this and one approach is called “stimulation”, where the reservoir pressure is increased by some means, one of which is water injection. This method involves injection of water directly into a particular reservoir to replace the lost oil and thus increase the reservoir pressure. As water is heavier than oil it does not easily mix with the oil and therefore sinks to the bottom of the reservoir (see water layer W in FIG. 2) allowing oil production to continue at an increased pressure.
Currently water injection wells are either specially drilled and created for this specific purpose or use a converted oil well. A schematic of such a well can be seen in FIG. 3. A separate water injection well is required for each well to enable the water supply to each to be controlled independently. If a well has more than one producing reservoir, the implementation of water injection therefore becomes significantly more complex and expensive.